1. Technical Field
The present invention relates to a method of manufacturing an optical fiber base material. More specifically, the present invention relates to a method of manufacturing a base material for a low-loss optical fiber, which is being capable of constantly providing glass base material for an optical fiber containing little impurity such as heavy metal.
2. Description of the Related Art
VAD is well-known as a method of manufacturing base materials for optical fibers. For example, according to this method, glass particles in vapor phase are generated with a core deposition burner and a cladding deposition burner disposed in a reaction chamber; and the glass particles are deposited onto a tip of a starter attached to a shaft which rotatably lifts up, so that a porous base material for an optical fiber composed of a core layer and a cladding layer is manufactured.
Usually, GeO2 having a high refractive index is added to the core layer. Such porous base material for an optical fiber is dehydrated by heating it at approximately 1100 degrees Celsius in a certain atmosphere containing dehydrated gas such as chlorine, and then transparently vitrified by heating the dehydrated material at approximately 1500 degrees Celsius in a certain atmosphere containing inert gas such as helium, so that a core member for an optical fiber base material is obtained.
The heating furnace for dehydrating and transparently vitrifying has heaters in and around a furnace tube, and the furnace tube is shut out the air. In one arrangement, a heater is attached to a position around the center of the longitudinal direction of the furnace tube, and the porous base material is suspended in the furnace tube and moves through a heating region of the heater. Thus, the porous base material is dehydrated or transparently vitrified.
Here, the reason why the porous base material is suspended from above is that it is easy to keep the porous base material vertical by gravity. Therefore, it becomes easy to pass the porous base material through the center of the furnace tube, i.e. the center of the heating region. In addition, it is convenient that the porous base material is pulled up from above in order to appropriately define a length of the porous base material which is processed because a portion of the base material which is heated shrinks longitudinally when it is transparently vitrified.
Moreover, the reason why the porous base material is suspended from above is that the upper portion of the porous base material manufactured by VAD is easily gripped because the starter is attached to the upper portion. Consequently, the top of the furnace tube has an opening portion, the base material is taken in and out from the opening portion. While the base material is processed, the opening portion of the furnace is covered with a lid, so that the inside of the furnace tube is shut out the air. A silica tube made of natural quartz is often employed for the furnace tube because the natural quartz has high heat resistance and little impurity.
Meanwhile, gas constituting atmosphere within the furnace tube is introduced from the lower portion of the furnace tube and discharged from the upper portion of the furnace tube. The reason comes from the fact that since the gas heated in the heating region in the center of the furnace tube generates an ascending current, it is convenient that a stream of the introduced gas is also flowed upwardly so as not to unnecessarily convect the gas.
Thus, the porous base material is placed on upper than the heating region immediately after being introduced because the porous base material is introduced into the furnace tube from the upper portion thereof. Since a step of dehydrating starts at the position where the porous base material is placed on upper than the heating region, subsequent steps usually have been performed as pulling up the porous base material. For example, in Patent Document 1, a step of dehydrating and transparently vitrifying the porous base material is separated into a dehydrating step and a transparently vitrifying step in order to reduce residual chlorine in the base material. The steps progress the following order: inserting the porous base material into the furnace from above; dehydrating the porous base material progressively from the bottom end as falling at an appropriate speed; pulling the base material up into the furnace once after completing the dehydration; falling the base material at an appropriate speed again. Thus, the porous base material is transparently vitrified progressively from the bottom end thereof.
As described above, the direction in which the gas flows is generally opposed to the direction in which the porous base material moves during each step in the conventional art.
The core member manufactured by the above described steps is formed as an optical fiber base material by jacketing the core member with a silica tube, or further depositing a cladding on the outside thereof by another Vapor Phase Deposition to obtain a desired core to cladding ratio.
Patent Document 1: Japanese Patent Application Publication No. 61-270232
An optical fiber obtained by drawing the optical fiber base material manufactured by the above described method could infrequently have a larger transmission loss than usual. When the characteristic of the transmission loss is examined, a transmission region for the short wavelength 1310 nm is larger than a transmission region for the long wavelength 1550 nm. When it is examined more specifically, the peak of losses is found around 900 nm. Consequently, it is considered that the high transmission loss of the optical fiber is caused by contaminating the base material with vanadium when the porous base material is transparently vitrified. It is not preferable that such high transmission loss deteriorates the transmission characteristics of optical fibers.